The field of the invention is xe2x80x9call-opticxe2x80x9d converters to convert an input signal in the NRZ format to an output signal in the RZ format. It also relates to a conversion process.
All-optic transmission systems, and particularly wavelength division multiplexed WDM networks use different modulations and different data formats. Two standard data formats are fairly widespread. These are the Non Return to Zero (NRZ) format and the Return to Zero (RZ) format. As explained in page 359 of the manual entitled xe2x80x9cOptique et Txc3xa9lxc3xa9communicationxe2x80x94Transmission et traitement optique de l""information (Optics and Telecommunicationsxe2x80x94Transmission and optical information processing)xe2x80x9d by A. COZANET et al. Published by EYROLLES, 1983, in a Non Return to Zero system a xe2x80x9conexe2x80x9d level is transmitted by a high level throughout the duration of the bit transmission time, and a xe2x80x9czeroxe2x80x9d is transmitted by a low level during this duration; in a Return to Zero system, a xe2x80x9conexe2x80x9d is transmitted by a high level for part of the period, usually the first half of the transmission time of the bit followed by a low level, and a xe2x80x9czeroxe2x80x9d is transmitted by a low level throughout the duration of the bit.
This simple description shows that the number of pulses transmitted for a set of data will be greater for the RZ format than for the NRZ format. In an NRZ format, a continuous sequence of xe2x80x9conesxe2x80x9d will require a high level signal throughout the duration of the transmission of this sequence, namely one long pulse.
In the Return to Zero format, a sequence of xe2x80x9conesxe2x80x9d will be represented by a sequence of pulses. Similarly, an isolated xe2x80x9conexe2x80x9d in the RZ format will produce a pulse half as long as the length of the pulse in the NRZ format, assuming that the bit time is of the same duration for the two formats, such that the transmission of the RZ format will require a larger pass bandwidth.
For all these reasons, the pass band necessary to transmit the same data is twice as wide for the RZ format as for the NRZ format.
Therefore, it can be seen that the NRZ format does have an undoubted advantage compared with other formats that require a greater bandwidth for the same transmission speed.
However, the RZ format is also useful in some applications, for example multiplexing, demultiplexing by passive time division, soliton generation and deletion of the BRILLOUIN stimulated dispersion.
This is why converters from one format to the other are necessary to benefit from the advantages of both formats.
For example, one first known example of this type of converter is briefly described in an article in xe2x80x9cElectronics Lettersxe2x80x9d on Feb. 13 1992 (vol. 28, No. 4, pages 405, 406) written by A. D. Ellis and D. A. Cleland and entitled xe2x80x9cCommutation tout optique ultra rapide dans un miroir boucle optique non lineaire (NOLM) (Ultra rapid all-optic switching in a non-linear optical loop mirror)xe2x80x9d. As shown in FIG. 1 in this article, the device comprises a SAGNAC anti-resonance interferometer.
Inputs to the interferometer consist firstly of a regular pulse stream and secondly a signal in the Non Return to Zero format representative of a data signal.
The power of the two signals is chosen such that if only one of these signals is present, in other words is at the high level, a destructive interference occurs such that the signal at the output from the interferometer is low which corresponds to normal operation of a SAGNAC interferometer considering the phase delay between the signal in the forward direction and the signal in the reverse direction resulting from the birefringent nature of the fibre. However, when the two signals are high, which has the effect of doubling the total optical power, an additional phase of xcfx80 is caused by modulation of the high power signal propagating in the reverse direction and therefore by constructive interference at the output from the interferometer.
The device thus behaves like an xe2x80x9candxe2x80x9d gate.
Depending on the pass band of a filter located at the output from the interferometer, an RZ signal resulting from conversion of the NRZ signal can be obtained either on a carrier at the wavelength carrying the pulse stream, or at the wavelength carrying the NRZ signal.
A second embodiment of the NRZ-RZ converter is described in an article by S. BIGO, E. DESURVIRE, S. GAUCHARD and E. BRUN entitled xe2x80x9cAmxc3xa9lioration de dxc3xa9bit par conversion optique NRZ-RZ et multiplexage par division temporelle passive pour les systxc3xa8mes à transmission soliton (Improvement of flow by NRZ-RZ optical conversion and multiplexing by passive time division for soliton transmission systems)xe2x80x9d published in the xe2x80x9cElectronics Lettersxe2x80x9d journal Jun. 9, 1994 (vol. 30, No. 12, pages 984-985). In the following, we will only consider the NRZ-RZ conversion described in this article. As in the previous case, a non-linear optical loop mirror is used. Also as in the previous case, two signals are input into the loop forming a SAGNAC interferometer. Firstly, a control signal is input at a wavelength xcexc, and secondly an NRZ signal is input at wavelength xcexs. An interferometer polarization controller is adjusted to minimize the output signal when the control signal is low in order to obtain a SAGNAC interferometer. After amplification, the control signal is input into the loop forming an interferometer through an 80/20 coupler located close to the point of sharing between the forward wave and the reverse wave.
The authors report that with this passive loop, in other words a loop that does not comprise an optical amplifier in the loop as in the previous example, they can obtain an RZ signal after conversion of the input NRZ signal.
The pulse width of the RZ signal is also slightly less than the width of the control pulses at wavelength xcexc.
However, the difference in level between the low level and the high level is more than 20 dB, representing an improvement by a factor of 10 compared with the previous example with active loop.
Note that in the two examples described above, the fibre loop mirrors are very sensitive to the light polarization and temperature fluctuations.
A third example embodiment is mentioned in an article by David NORTE and Allan E. WILLNER entitled xe2x80x9cDxc3xa9monstration expxc3xa9rimentale d""une conversion tout optique entre des donnxc3xa9es aux formats RZ et NRZ incorporant des changements non inverseurs de longueur d""onde et conduisant xc3xa1 une transparence du format (Experimental demonstration of an all-optic conversion between data in the RZ and NRZ formats including changes that do not invert the wavelength and leading to format transparency)xe2x80x9d published in xe2x80x9cIEEE photonics technology Lettersxe2x80x9d (vol. No. 8, No. 5, May 1996, pages 712-714).
The device mentioned in this article applies to an RZ-NRZ converter according to the diagram shown in FIG. 1 of this article. The description of the device and its operation is not very clear because an optical amplifier SOA1 mentioned in the text is not shown in the figure. However, it can be understood that the first step is to recover the clock signal from the RZ signal. It is also explained that a device not described is used to change from an NRZ format to an RZ format such that either of the two formats can be used.
Each of the devices described or mentioned in these articles are subsystems that include several opto-electronic or electronic devices and require assembly work.
As mentioned above, devices based on SAGNAC interferometers are sensitive to fluctuations in temperature and light polarization. All devices described in these articles require a clock signal or a clock recuperation.
Therefore, there is a need for an NRZ-RZ conversion device that is easy to design and make.
The basic idea of the invention is to use an interferometric structure, for example a Mach-Zehnder structure with two arms (a first arm and a second arm). At least the first or the second arm comprises an element in which the optical index can vary as a function of the optical power present in this element, for example a semi conducting optical amplifier (SOA).
An NRZ signal (Se) is injected on one input of the interferometer, such that this signal is distributed in each of the interferometer arms in a balanced manner. If no control signal is input into an arm in which the index varies with optical power, the input signal(Se) is unchanged on one of the interferometer outputs. However, if a control signal (Sc) is input into this arm of the interferometer and if the level of this control signal (Sc) is well chosen, an index change exactly equal to the quantity necessary to put the optical signals output from the two arms of the interferometer into phase opposition, in other words with a phase shift of (2k+1)xcfx80 with respect to each other, can be obtained in the element for which the optical index varies as a function of the optical power present in this element. In this instantaneous configuration, the two signals output from the input signal (Se) present in each of the arms of the interferometer cancel each other at the output from the interferometer. Therefore for the input signal (Se), the interferometer behaves like an optical gate that is opened by a control signal (Sc). According to the invention, this phenomenon is used to convert a signal from the NRZ format into a signal in the RZ format, without using an optical clock signal. This is done by inputting the signal at the output from the interferometer into one of the arms of the structure, while applying a time delay to it approximately equal to half of the time bit of the NRZ signal. The delayed output signal is thus used as a control signal to close or open the optical gate described above. The time delay removes the optical power in the interferometer output signal for a time. As will be seen in more detail later, the result is an output signal corresponding to the input signal but in the RZ format.
Thus, the invention is related to a converter to convert a signal in the NRZ format with a bit duration of T, into a signal in the RZ format, characterized in that it comprises:
an interferometric structure with two arms, a first arm and a second arm, each of the arms having two ends, a first end and a second end, at least one of the arms of the interferometer containing a medium for which the value of the optical index is variable as a function of the optical power passing through the said medium, this interferometric structure having at least one input and at least one output
one of the inputs being intended to receive the NRZ signal to be converted, one of the outputs carrying a signal resulting from interference between a signal present in the first arm and a signal present in the second arm:
the converter also comprising:
coupling means for coupling to an output from the interferometric structure, and an arm of the said structure containing the medium for which the value of the optical index is variable as a function of the optical power
delay means placed between the said output from the interferometric structure and the said coupling means, the said delay means causing a delay equal to approximately T/2.
Preferably, the active media for which the refraction index is sensitive to the power that passes through them are composed of active layers of a semi conducting optical amplifier.
The time bit for high transmission speeds is very short (of the order of the few tens of a picosecond) and, for example, the delay input to the output signal reinput into one of the arms can be achieved by means of an optical guide integrated on a substrate of the interferometer or a component, for example an optical amplifier that also performs the function of adjusting the level of the output signal before reinputting it into one of the arms of the interferometric structure.
The output signal from the interferometric structure may be input onto the end of the arm of the structure into which the NRZ signal to be converted is also input, such that the two signals propagate in this arm in the same direction. In this case, it will be preferable to convert the signal output from the structure into a wavelength to prevent interference between these two signals occurring in the said arm.
The output signal from the interferometric structure can also be input to one end of the arm of the structure opposite to the end into which the NRZ structure is input. In this case, it may or may not be possible to convert the output signal from the interferometric structure into a wavelength before it is input into the said arm of the structure.
Preferably, if there is no signal from the output from the structure, the optical paths of the two arms of the structure are equal, thus creating constructive interference between the signals present in each of the arms.
Preferably, means placed between the output from the structure and one of the arms adjust the optical power level of the signal at the output so as to modify the optical path of one of the arms compared with the other to obtain destructive interference.
The invention also relates to a process for conversion of a signal in the NRZ format with a bit duration T into a signal in the RZ format characterized in that:
the signal to be converted is input into each of the arms of an interferometric structure, each of the arms of the structure containing an optical medium for which the value of the index is variable as a function of the optical power passing through the said medium,
a signal present at the output from the said structure is input into only one of the arms of the said structure, delayed by a value equal to about half a bit duration,
the RZ signal resulting from the conversion of the NRZ signal is collected at the said output.